[llvm] [SCEV] Add non-poison/non-zero checks on denominators (PR #117152)

[Florian Hahn via llvm-commits]

================
@@ -3422,9 +3422,14 @@ const SCEV *ScalarEvolution::getUDivExpr(const SCEV *LHS,
   if (const SCEV *S = UniqueSCEVs.FindNodeOrInsertPos(ID, IP))
     return S;
 
+  // If the denominator is zero, the udiv will trap.
+  auto IsValidDenominator = [&] {
+    return isGuaranteedNotToBePoison(RHS) && isKnownNonZero(RHS);
----------------
fhahn wrote:

Share logic with the existing code in `getSequentialMinMaxExpr`

```
bool MayBeUB = SCEVExprContains(Ops[i], [this](const SCEV *S) {   
      auto *UDiv = dyn_cast<SCEVUDivExpr>(S);
      // The UDiv may be UB if the divisor is poison or zero. Unless the divisor
      // is a non-zero constant, we have to assume the UDiv may be UB.
      return UDiv && (!isKnownNonZero(UDiv->getOperand(1)) ||
                      !isGuaranteedNotToBePoison(UDiv->getOperand(1)));
    });

```

https://github.com/llvm/llvm-project/pull/117152